Walgreens Boots Alliance (WBA) stock tumbled Thursday after the pharmacy chain slashed its dividend by almost half. The move puts the company at risk of being removed from the S&P 500 Dividend Aristocrats, an index of S&P 500 companies that have raised their dividends for at least 25 consecutive years. At its old dividend level, Walgreens was one of the stocks with the highest dividend yields in the S&P 500. But that's about to change.The company announced a quarterly dividend of 25 cents a share on Thursda ...